[Xen-devel] [PATCH 1/3] tests/x86emul: Helpers to save and restore FPU state

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Introduce common helpers for saving and restoring FPU state.  During
emul_test_init(), calculate whether to use xsave or fxsave, and tweak the
existing mxcsr_mask logic to avoid using another large static buffer.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
---
 tools/tests/x86_emulator/x86-emulate.c | 70 +++++++++++++++++++++++++++-------
 tools/tests/x86_emulator/x86-emulate.h |  4 ++
 2 files changed, 60 insertions(+), 14 deletions(-)

diff --git a/tools/tests/x86_emulator/x86-emulate.c 
b/tools/tests/x86_emulator/x86-emulate.c
index 9056610..47e503d 100644
--- a/tools/tests/x86_emulator/x86-emulate.c
+++ b/tools/tests/x86_emulator/x86-emulate.c
@@ -26,26 +26,68 @@
 
 uint32_t mxcsr_mask = 0x0000ffbf;
 
+static char fpu_save_area[4096] __attribute__((__aligned__((64))));
+static bool use_xsave;
+
+void emul_save_fpu_state(void)
+{
+    if ( use_xsave )
+        asm volatile ( "xsave" __OS " %[ptr]"
+                       : [ptr] "=m" (fpu_save_area)
+                       : "a" (~0ull), "d" (~0ull) );
+    else
+        asm volatile ( "fxsave %0" : "=m" (fpu_save_area) );
+}
+
+void emul_restore_fpu_state(void)
+{
+    if ( use_xsave )
+        asm volatile ( "xrstor" __OS " %[ptr]"
+                       :: [ptr] "m" (fpu_save_area), "a" (~0ull), "d" (~0ull) 
);
+    else
+        asm volatile ( "fxrstor %0" :: "m" (fpu_save_area) );
+}
+
 bool emul_test_init(void)
 {
+    union {
+        char x[464];
+        struct {
+            uint32_t other[6];
+            uint32_t mxcsr;
+            uint32_t mxcsr_mask;
+            /* ... */
+        };
+    } *fxs = (void *)fpu_save_area;
+
     unsigned long sp;
 
-    if ( cpu_has_fxsr )
+    if ( cpu_has_xsave )
     {
-        static union __attribute__((__aligned__(16))) {
-            char x[464];
-            struct {
-                uint32_t other[6];
-                uint32_t mxcsr;
-                uint32_t mxcsr_mask;
-                /* ... */
-            };
-        } fxs;
-
-        asm ( "fxsave %0" : "=m" (fxs) );
-        if ( fxs.mxcsr_mask )
-            mxcsr_mask = fxs.mxcsr_mask;
+        unsigned int tmp, ebx;
+
+        asm ( "cpuid"
+              : "=a" (tmp), "=b" (ebx), "=c" (tmp), "=d" (tmp)
+              : "a" (0xd), "c" (0) );
+
+        /*
+         * Sanity check that fpu_save_area[] is large enough.  This assertion
+         * will trip eventually, at which point fpu_save_area[] needs to get
+         * larger.
+         */
+        assert(ebx < sizeof(fpu_save_area));
+
+        /* Use xsave if available... */
+        use_xsave = true;
     }
+    else
+        /* But use fxsave if xsave isn't available. */
+        assert(cpu_has_fxsr);
+
+    /* Reuse the save state buffer to find mcxsr_mask. */
+    asm ( "fxsave %0" : "=m" (*fxs) );
+    if ( fxs->mxcsr_mask )
+        mxcsr_mask = fxs->mxcsr_mask;
 
     /*
      * Mark the entire stack executable so that the stub executions
diff --git a/tools/tests/x86_emulator/x86-emulate.h 
b/tools/tests/x86_emulator/x86-emulate.h
index a1c4774..45acea4 100644
--- a/tools/tests/x86_emulator/x86-emulate.h
+++ b/tools/tests/x86_emulator/x86-emulate.h
@@ -52,6 +52,10 @@ extern uint32_t mxcsr_mask;
 #define MMAP_SZ 16384
 bool emul_test_init(void);
 
+/* Must save and restore FPU state between any call into libc. */
+void emul_save_fpu_state(void);
+void emul_restore_fpu_state(void);
+
 #include "x86_emulate/x86_emulate.h"
 
 static inline uint64_t xgetbv(uint32_t xcr)
-- 
2.1.4


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